This invention relates to an antenna apparatus mainly used with a portable radio and in particular to an antenna apparatus being contained in a portable radio for providing a good radiation characteristic even in a state in which a portable radio is brought close to a human body for use.
In recent years, a demand for mobile radios such as portable telephones has been sharply growing, and a compact, lightweight, and slim radio has been required. Thus, hitherto, a fixed-type helical antenna, a plate-like inverse F antenna, etc., has been used as an antenna and a small-sized antenna system which has good portability and which will not cause an inconvenience when it is used with a small-sized radio is provided.
FIG. 19 is an external view of a fixed-type helical antenna widely used as a portable telephone antenna in a related art. A fixed-type helical antenna element 21 is placed on a portable telephone main unit 20, whereby a compact and lightweight antenna system is provided.
FIG. 20 shows the structure of a plate-like inverse F antenna widely used as an internal antenna of a portable telephone in a related art. The antenna is able to be housed in a portable telephone main unit 20 and can be placed close to the top of a radio base plate. As the antenna, a radiation element 22 is placed close in parallel with a radio base plate 23, a part of the radiation element 22 is grounded to a ground point 24, and power is fed into a part from a feeding point 25, whereby a low-profile antenna is provided and it is made possible to design a portable telephone with an antenna not protrude the portable telephone main unit.
However, with both the fixed-type helical antenna in FIG. 19 and the plate-like inverse F antenna in FIG. 20, much ground current flows not only to the antenna element, but also onto the radio base plate and when the radio is brought close to a human body for use, the antenna is affected by the hands and the head and the gain is degraded largely; this is a problem.
FIG. 21 is a current distribution drawing of the fixed-type helical antenna in the related art. In FIG. 21, wire 26 approximates the radio base plate and the antenna element and an absolute value distribution 27 of current flowing onto the wire 26 when power is fed into the antenna is represented three-dimensionally. It is also seen in the figure that much ground current flows not only onto the helical antenna, but also onto the radio base plate.
FIG. 22 shows a characteristic representing the radiation directivity of the fixed-type helical antenna in the related art. As a result of large ground current flowing not only onto the antenna, but also onto the top of the radio base plate, a xcex8 component is dominant. Consequently, in a state in which a human being carries the radio and tilts it for use, the polarized wave of an arrival wave from a base station does not match that of the radio antenna and the reception performance largely degraded; this is a problem.
Further, if each of the antennas is miniaturized and is placed in the radio main unit, it is affected by peripheral parts and the radio base plate and becomes a narrow band and the gain is degraded largely; this is a problem.
It is therefore an object of the invention to realize a balanced system antenna wherein the current component flowing onto a radio base plate is decreased and the gain is less lowered if the antenna is brought close to a human body for use, and provide a small-sized, wide-band, and high-gain antenna apparatus which can operate in a wide band if it is installed close to a radio base plate and can form radiation directivity responsive to an arrival wave.
According to the invention, there is provided an antenna apparatus being contained in a portable radio main unit, the antenna apparatus comprising a loop antenna element shaped like a rectangle with the ratio between a short side and a long side being 10 or more, wherein the loop antenna element has an outer peripheral length which is roughly the same as one wavelength at a first frequency and is placed close in parallel to a radio base plate with a sufficiently small spacing as compared with the wavelength and further is turned up so that the short side is brought close to the feeding section side.
Thus, a current distribution concentrates on the loop antenna element, the current component flowing on the top of the radio base plate can be lessened, and the effect of a human body can be decreased. Further, the antenna element is turned up, whereby it can be miniaturized while it has a wide-band characteristic although the antenna element is placed extremely close to the top of the radio base plate.
The current distribution of the short side of the loop antenna element is zero, so that the current components brought close in parallel do not cancel out each other and highly efficient operation can be performed; the small-sized, high-gain antennal apparatus can be provided.
Since the loop antenna element is connected to the balanced feeding line, the current distribution can be concentrated stably on the loop antenna element
One or more passive elements are placed with a sufficiently small spacing as compared with the wavelength along the loop antenna element, so that the antenna apparatus can be provided with a wide-band characteristic and can receive stably in a wide band.
The passive element has a resonance frequency different from the first frequency, so that the antenna apparatus can be provided with a double-resonance or triple-resonance characteristic and can receive at a plurality of frequencies or in a plurality of systems.
A part or the whole of the loop antenna element or the passive element is shaped like a plate, so that the band is further widened and the antenna apparatus can receive stably in a wide band.
The loop antenna element or the passive element is formed on a structure of resin, ceramic, or a printed circuit board, so that a solid and stable antenna system can be provided.
The ratio between a current flowing onto the top of the loop antenna element and a high-frequency current flowing onto the top of the radio base plate is changed, so that the optimum radiation directivity can be formed in response to change in the operating environment or arrival radio wave, and a highly sensitive antenna system can be provided. As means for changing the high-frequency current ratio, adjustment means for providing a phase difference between high-frequency signals supplied from the balanced feeding line can be provided or the loop antenna element or the passive element is asymmetrical with respect to the feeding section.